1. Field of the Invention
The field of the present invention relates to power amplification and, more specifically, to apparatus and methods for amplifying electronic signals such as audio signals.
2. Background
With a conventional power amplifier, the maximum available output power is generally limited by at least two factors: the voltage swing available at the amplifier's output, and the load impedance. The voltage swing is itself typically limited by the amplifier rail voltage. In the case of a car audio amplifier for instance, the rail voltage is the nominal 14.4 Volts of the car battery; thus, if the amplifier were able to swing all the way up to the power rails, it could deliver ˜7 Volts peak output. This peak voltage is only sufficient to deliver about 3 Watts of power to an 8Ω load. To obtain greater output power, the load impedance can be reduced (for example, a 1Ω load would allow 25 Watts of output power), but in order not to encounter significant losses in the wiring, the cables need to be thicker and heavier.
In general, lowering the load impedance on an amplifier will increase the current that has to be supplied by the amplifier and increase the amplifier distortion as well as requiring more expensive output devices. Thus, merely lowering the load impedance by itself may not provide additional power. It would therefore be advantageous to be able to increase the voltage output available from the amplifier so that, for example, a higher load impedance can be employed and/or greater power be delivered to a load.
One technique for increasing output voltage is known as bridging, whereby two anti-phase amplifiers are used with the load tied between their outputs. This approach can double the available output swing and for a given load impedance, which would quadruple the output power. Even so, the maximum available power to a 1Ω load, using the typical power supply conditions described above, would be limited to approximately 100 Watts, and would still have the complication of a low impedance load. However, this simplified analysis hides the fact that under these conditions, each amplifier is not only having to supply twice the output current as compared to the non-bridged condition, but also sees half the effective load impedance. In practical applications, the amplifiers may not be able to supply the required current.
A more valid comparison might be to calculate the maximum output that can be achieved for a given maximum output current capability. In the case of the bridge amplifier, the load impedance would therefore be double that used for the single amplifier example. With twice the load resistance, the power into the load is only twice that of the single amplifier. The effective load impedance seen by each half of the bridge amplifier is now the same as the single amplifier example and given that the load current sourced by the amplifier is the same, then the amplifier distortion is the same also.
For applications in which the amplifier is driving a speaker load for audio sound reproduction, the bridge amplifier could be replaced by two single amplifiers, each driving one of a pair of voice coils on a dual voice coil loudspeaker. The overall output level from the loudspeaker would then be identical for the bridge amplifier and the two single amplifiers.
Another technique to increase effective power output is to employ a switching power supply to raise the power supply voltage to the amplifier. The amplifier output voltage capability is thereby increased, allowing a higher load impedance to be used for a given output current capability. This approach can mitigate the need for a low load impedance. However, a switching power supply typically operates at high frequency and high power and needs careful design to avoid interference.
Another technique, which has been employed, for example, by Philips Semiconductors of Eindhoven, the Netherlands, in its TDA1560/1562 products, involves an amplifier system that modulates its own supply voltage in order to provide a higher output voltage swing. A bridge amplifier is used in conjunction with a capacitive booster circuit that lifts the supply to approximately twice the steady-state level when a larger output voltage swing is required. Under quiescent conditions, the output of each amplifier in the bridge sits at approximately half the battery voltage. As the output of one half of the bridge amplifier approaches the positive supply rail, the booster circuit begins to lift the supply voltage so that the amplifier does not clip. However, at the same time, the output of the other half of the bridge amplifier approaches ground. The output can go no lower than ground potential, and so the amplifier system adds the difference between the actual negative going output and the desired negative going output to the positive going output at the other side of the bridge amp. This operation can result in significantly distorted waveforms at each individual output of the bridge amplifier, but the output measured differentially across the two outputs is generally linear (within the limitations of the correction circuitry). The booster circuit adjusts to maintain sufficient headroom above this modified output waveform. Thus the bridge amplifier output is able to increase sufficiently to give a peak output voltage close to twice the battery voltage, at the expense that this technique can generally only be used in bridge amplifier mode.
While the foregoing approaches may increase the effective output capability of an amplifier, they still have appreciable limitations. It would therefore be advantageous to provide an improved audio amplification apparatus or method which overcomes one or more of the foregoing problems or limitations, provides increased power output when needed, and/or provides other benefits and advantages.
Another approach that has been developed for providing temporary increased power output when needed is disclosed in U.S. Pat. No. 7,834,702, entitled “Efficient Power Amplifier,” assigned to the assignee of the present invention. That patent generally describes a variety of embodiments of a tracking power supply that temporarily raises the power rails to an amplifier when the output voltage approaches the nominal rail value. While this patent represents an improvement over the state of the art in a variety of respects, there may be situations where even greater voltage levels are required.
It would therefore be advantageous to provide an improved audio amplification apparatus or method which has an ability to provide greater voltage boosts when needed. It would further be advantageous to provide an efficient audio amplification apparatus or method which provides increased power output without the need for a bridge amplifier or a switching power supply.